AIKIDO-2026-623572

TL;DR

The soupsieve CSS selector parser trims trailing whitespace and comments with the RE_WS_END regex. That pattern has no start anchor and is applied with search(), so the engine retries at every offset. A selector with a long run of whitespace or repeated comments before a token then takes quadratic time, so a roughly 20 KB selector consumes about ten seconds of CPU. The fix rewrites trailing trimming as a start anchored match over the reversed string so trimming stays linear.